1. Field of the Invention
This invention relates to a frame for a bicycle including a single main frame, a seat tube, two chain stays and rear forks, and particularly to a seat tube frame consisting of a seat tube and a hanger portion or bottom bracket integral with the seat tube.
2. Description of the Prior Art
With hitherto used bicycles, seat tubes and hanger portions or bottom brackets have been separately made in separate parts. After the seat tube and the bottom bracket have been integrally combined as by welding, the bottom bracket is formed in its inner surface with screw-threads with which a cup and a retainer are provided. Such a production process is, therefore, complicated and thus increases production cost.
With the prior art bicycles, moreover, there have been a great number of parts of bicycle frames. Furthermore, the parts have been combined by welding or wrapping casting (joining a member to a cast article wrapping thereabout in casting), which makes the production process complicated and thus lower the production output. What was worse still, heating by welding adversely affects the strength of the frame.